Chemical vapor deposition (hereafter, “CVD”) is a conventionally used process for producing high-purity, high-performance materials, such as thin films on semiconductors or growing crystalline structures. Deposition of the films includes exposing a substrate to volatile chemicals, i.e., precursors, which react and/or decompose at a surface of the substrate.
The use of CVD for metal deposition, e.g., organometallic CVD or MOCVD, includes a metal atom (for example, but not limited to, Mo, Ta, Ti, W, Ru, Cu, Pt, and Pd) bonded to organic ligands. However, the CVD process has limitations in that internal structures or surfaces, with tortuous features, are not effectively coated.
Supercritical chemical fluid deposition (hereafter, “SFD”) is one conventional solution that is capable of depositing a metal coating onto a complicated surface/feature structure. During a SFD process, a supercritical fluid (substances at a temperature and pressure above a critical point (in a phase diagram) such that distinct gas and liquid phases do not exist), also referred to as the working fluid, is used as a solvent to the organometallic precursor. There are many supercritical fluids available for SFD process, but the most convenient may be carbon dioxide. The liquid-like state of the supercritical fluid enables increased solubility of the organometallic precursor, and the gas-like state of the supercritical fluid enables a deep, conformal penetration of the features of the substrate.
SFD processes have conventionally been performed in a hot-wall processing system 10, an example of which is shown in FIG. 1. The hot-wall processing system 10 includes a processing chamber 12 enclosing a processing space 14 that is heated externally. A substrate 16 and an organometallic precursor 18 are added to the processing chamber 12 and sealed. A working fluid (represented by arrows 20) is added, for example, via an injection system 22, and the processing space 14 and heated until the temperature and pressure required for the supercritical state of the working fluid is exceeded. The organometallic precursor 18 dissolves in the supercritical working fluid within the process space 14. A reducing agent, usually hydrogen, is then introduced to cause the metal portion of the organometallic precursor 18 to deposit onto the substrate 16. However, the metal portion is also deposited on other, interior surfaces of the processing chamber 12.
While the hot-wall processing system 10 of FIG. 1 is effective at coating substrates 16, the process is wasteful in that it deposits metal on all surfaces within the processing chamber 12. An alternative to the hot-wall processing system 10 is a cold-wall processing system 30, which is shown in shown in FIG. 2. The cold-wall processing system 30 places a substrate 32 on a heated pedestal 34 within the processing space 36 of the processing chamber 38. With the substrate 32 and an organometallic precursor 40 in place, the processing chamber 38 is sealed and evacuated. A working fluid (represented by arrows 42) is added, for example, via an injection system 44, and the processing chamber 38 is heated until the supercritical state of the working fluid is surpassed. The organometallic precursor 40 dissolves in the supercritical working fluid, and then the reducing agent (again, usually hydrogen) is added. To prevent deposition of the metal component onto all interior surfaces of the chamber 38 (like the aforementioned hot-wall processing system 10 (FIG. 1)), the organometallic precursor 40 should be thermally stable, stable to hydrogen reduction at lower temperature, and yet able to be reduced at elevated temperatures. The pedestal 34 of the cold-wall processing system 30 is heated such that deposition of the metallic portion is on the substrate and pedestal. Thus, the cold-wall processing system is more efficient than the hot-wall process 10 (FIG. 1), and is particularly useful for depositing copper and ruthenium coatings.
However, the cold-wall processing system 30 has not been conventionally used to deposit noble metals because conventional noble metal precursors were not stable under cold-wall SFD processing conditions. As such, there remains a need for noble metal precursors that would undergo SFD using a cold-wall processing system.